A wind turbine comprises a tower, a nacelle and a wind turbine rotor with a hub and rotor blades. The wind turbine comprises an electric generator. The electric generator comprises a generator rotor and a generator stator. The hub with the rotor blades is connected to the rotor of the electric generator.
The nacelle of the wind turbine comprises a support structure. The rotor is attached to the support structure and the support structure is connected to the tower of the wind turbine. The support structure supports the generator and the rotor of the wind turbine on the tower.
The wind interacts with the rotor blades and causes a rotation of the hub and the rotor of the electric generator.
The wind induces loads and vibrations into the wind turbine. The loads and vibrations are transferred from the rotor of the wind turbine over the support structure to the tower.
The supports structure is prepared to transfer the loads and vibrations from the rotor of the wind turbine to the tower. The support structure is planned to last as long as the lifetime of the wind turbine. In this case a lifetime of more than 20 or 25 years is normal.
Thus the support structure needs to be planned rigid enough and needs to be machined with a minimum of faults and tolerances.
The support structure is usually a casted piece of metal. A support structure normally weighs between 10 and 60 tons. Metal pieces of the size of a support structure of a modern wind turbine are difficult to cast. Contraction cavities are a common problem that might occur in casted support structures of this size. In addition the support structure needs to be machined afterwards.
Over the last decade the planned maximum output power of wind turbines increased. In parallel the rotor diameter and the hub height of the wind turbines increased. Thus also the capabilities of the support structure needed to increase. The support structures were planned bigger and more massive.
The components of a wind turbine are normally manufactured in a production site and are then transported to the installation site of the wind turbine. Usually road transport is necessary for at least a part of the way. The size and weight of components that can be transported on a road are normally limited.
Support structures of a weight of more than 30 tons are rather difficult to transport. Thus the size of the support structure is limited. Thus the maximum size of a wind turbine is limited due to the possibilities for road transport.